Include iotop in the node-image

We don't currently package iotop, but there have been requests to
add it, and it's a very small addition to the iamge. Include it.

Change-Id: I7b77b08bfc5083664162289d402fc460aa66426f
Bug-url: https://bugzilla.redhat.com/show_bug.cgi?id=1052348
Signed-off-by: Ryan Barry <rbarry@redhat.com>